Climate impact and life cycle cost assessment

A simplified life cycle assessment and life cycle cost estimation, that covered the most important steps of the life cycles, were done for the oxygenation pumps and precipitation techniques.

Four techniques to remove phosphorus in or into the Baltic Sea were investigated in six scenarios. Three scenarios was oxygen pumping into the sea with the secondary effect of phosphorus turnover and removal, either by the WEBAP I, wave-energized aeration pump, or by an electric pump (WEBAP II, diesel aggregate off shore, or electricity near shore). Three scenarios consisted of precipitation, either in wastewater treatment plants (WWTP) or by spreading of treatment chemical in the sea.

The goal was to compare climate impact and costs for several P-removing techniques, taking into account materials production, transports, operation and maintenance of these. No consideration was taken to end-of-life treatment of the techniques since it was outside the scope of the study. However, the precipitation from the chemical treatment techniques was assumed safely taken care of, and the pumps used for materials recycling by end-of-life.

Removal of 1 kg of phosphorus was chosen as the basis for comparison (functional unit). Emissions from materials and chemicals production, as well as transports and energy use were normalized to removal of 1 kg P. All costs were also normalized to removal of 1 kg phosphorus

For production of techniques, PAX has by far the largest climate impact. Impacts from fuel use or electricity production are the largest share of the electric pump’s life cycle. The wave-energized pump can compete with impacts from FeSO4 treatment, and is efficient regarding removal of P during lifetime.

Figure. Life cycle cost of the different techniques.

The results for the costs follow mainly the same distribution as climate impact (except for a switch in ranking between PAX off shore and electric pump diesel off shore). The wave-energized pump has very low life cycle cost compared to other techniques, which means efficient P removal for the money invested.

It is important to note that the techniques differ in their potential. For the Swedish case, removal of phosphorus in all major treatment plants is already performed with a maximum efficiency, and further removal of phosphorus can be complicated, or very costly. For example, to use extra sand filters to remove additional phosphorus, the marginal cost would be 4600 SEK/kg P. The pumps however, have a more unlimited potential since a large number of pumps can pump as much oxygen-rich sea water as there is at the sea surface (not without cost or climate impact though).

It is also a difference between focuses of the treatments. The plant-based treatments aims for eliminating inflow of phosphorus to the Baltic Sea, while the pumps focus on fixing a problem caused by phosphorus inflow. Therefore these techniques could be used in concert. The pumps could be useful for a period of time to re-oxygenize the Baltic Sea, while efficient P removal in treatment plants must continue so as not to cause oxygen depletion again.

Also note that if end-of-life treatment of the techniques were included, the climate impact and costs may look different. There could also be other environmental issues related to treatment of precipitate, or materials recycling of construction material.

Conclusively, the large offshore wave-driven pump WEBAP I, has in this estimation shown to be competitive with other techniques regarding climate impact and costs. It could therefore be a realistic option for phosphorus remediation of the Baltic Sea.